4.1: Neurons: The Building Blocks of the Nervous System

The nervous system is made up of cells like all parts of the body. The nervous system has two general types of cells. The neuron is the first type of cell. We will focus mostly on neurons. the cells that send electrical messages. The other types of cells are the glial (GLEE-uhl) cells. The glial cells help the neurons do their jobs. You have already learned about one type of glial cell, the astrocytes that help form the blood-brain barrier.

Neurons come in many forms, but they all have certain basic parts. Each neuron has a cell body, which contains its nucleus and other important cell parts. Each neuron has many branch-like projections called dendrites. The dendrites carry messages into the cell body. Each neuron has one projection called the axon to carry messages away from the cell body. Axons can be very short or very long. For example, the cell body of the neuron that sends the message to wiggle your little toe is located in your lower spinal cord, which is only about halfway down your back. So the axon of this neuron reaches down your back, down your leg, through your foot, and to the muscles in your little toe. There are structures at the end of an axon that send messages on to the next cell, which might be a muscle cell, a gland cell, or another neuron. Figure 3.1 shows the structure of a typical neuron.

Figure 3.1 A neuron (nerve cell) is usually microscopic. It has a shape like a chain of hot dogs with a hairy head and several hairy tails.

As you learned, there are two general types of cells in the nervous system-glial cells and neurons. But each type has many specialized kinds of cells that perform different functions. For example, you can organize neurons into three basic groups according to what they do. Sensory neurons send information collected by sensor cells into your central nervous system. Motor neurons bring instructions from the central nervous system back to your muscles and organs, telling them what to do. Interneurons make connections between sensory and motor neurons. Most neurons in the brain are interneurons.

Neurons are usually microscopic. So you might ask, “What are the string and thread-like fibers we see when all tissue is removed except the nervous system?” These fibers are nerves and they contain the axons of many neurons. Nerves are bundles of individual information pathways. In many ways these bundles are like telephone cables that contain the wires going to many, many different telephones.

Figure 3.2 If you imagine a neuron the size of a bedroom, you can get a better idea of a neuron's structure. The largest part of a neuron is the cell body with many dendrites where messages from other neurons come in. A long axon coated by protective glial cells extends from the cell body. At the end of the axon are nerve endings, which send messages out to other neurons.

Did You Know?

Each 16.4 cubic cm (1 cubic inch) of your brain has about 16,000 kilometers (10,000 miles) of axons in it. If you took all the neurons in your brain and placed them end to end, you could send nerve impulses up to the moon and back again. Interneurons may receive messages from a thousand other neurons, and may send their messages to an equal number of other neurons. But each motor neuron usually sends its axons to a single muscle fiber.

Imagine a neuron the size of a bedroom. A neuron that large would look something like a giant octopus with one very long tentacle. Look at Figure 3.2. The large, central part of the cell is the cell body. Remember this imaginary neuron is as big as a bedroom. Inside the cell body, you would find a nucleus the size of a tennis ball. The short tentacles are dendrites. The long tentacle, the axon, could probably stretch a mile. In a neuron the size of a bedroom, the axon would look like a cable with the diameter of a garden hose. The end of the axon has many little branches, somewhat like the unraveled end of a rope. Each little branch has a nerve ending that makes a synapse with another cell across a gap. In most cases, synapses are made between the nerve endings of one neuron and the dendrites or cell body of another neuron. The nerve endings of a motor neuron, like the one that causes your little toe to wiggle, make contacts with muscle cells.

Building a 3-D Model of a Neuron

Use your creativity to design and build a three-dimensional model of a neuron. Your model will need to represent the cell body, axon, and dendrites.

Dendrites act as the neuron's receivers or antennas. They receive messages from other nerve cells and send the messages toward the cell body. Dendrites get messages in several ways. Some dendrites are sensitive to temperature, pressure, touch, or chemicals. Some dendrites receive messages from specialized sensor cells such as cells in your muscles that act as “stretch sensors.” But usually dendrites get messages from other neurons. These messages travel as chemicals from a nerve ending at the end of an axon, across a synapse, to a dendrite where there are receptors for the neurotransmitters. A neuron may have more than 10,000 synapses on all its dendrites.

The cell body gets messages from synapses at its dendrites and on the cell body itself. The cell body then decides what the neuron will do. If the cell decides to send a message, it generates a nerve impulse, which is an electrical message. Then the electrical message travels down the axon toward synapses with other cells.

A nerve impulse is an electrical signal. But the electrical signal cannot cross the tiny gap between axon and dendrite at a synapse without help. The neurotransmitters bridge the gap between the axon and dendrite at the synapse. Imagine that a nerve impulse reaches the end of the axon at the synapse. At that point, little sacks containing neurotransmitter molecules open and spill into the gap. Then the chemical messages can cross the gap and link up with receptors on the membrane of the receiving cell. When the link is made between neurotransmitter and receptor, the receiving cell gets the message and can pass it along.

You have read about neurons being like a big bedroom with tentacles and a nerve impulse being like an electrical signal. These explanations are analogies, and they help us picture the activities of these parts of the nervous system. What other analogies can you think of to help you know more about neurons? What analogies can you think of to describe a nerve impulse? Draw pictures to illustrate your analogies.

What Is a Nerve Impulse?

We have described a nerve impulse as an electrical signal. But it is not like the electrical signals that travel down telephone wires or computer cables. Those electrical signals are extremely fast. They are so fast that they allow you to have a telephone conversation with someone on the other side of the earth.

Figure 3.3 A neuron receives information from other neurons. The cell body decides to pass the message along, and the axon transmits the message to the nerve endings.

Nerve impulses travel much slower. The nerve impulses are electrically charged atoms (ions) that pass back and forth across the membrane of the axon. You can think of a nerve impulse moving down an axon as something like a “human wave” in a football stadium. To form a “human wave,” one person stands up and sits down. Then the next person stands up and sits down, and so on. In the cell body, at the base of the axon, the neuron starts a nerve impulse by opening paths, or channels, in the membrane. These channels allow electrically charged atoms to move across. After this event occurs, the same thing happens in the next piece of axon, repeating all the way to the axon's end.

Did You Know?

A nerve impulse can travel 1 to 130 meters per second (2 to 280 miles per hour). The reflex neurons send the fastest impulses. Neurons that carry information about muscle aches or the way your clothes feel against your skin send the slowest impulses.

How fast is a nerve impulse in comparison to a “human wave?” The fastest axons send their impulses at about 100 meters per second. At that speed, a nerve impulse travels about the length of a football stadium in one second, much faster than the “human wave.” Some axons, such as those that carry messages about warmth, cold, and aches, are slower. They send their impulses at about 1 meter per second. It would take about 100 seconds for one of these axons to send an impulse the length of the football stadium. That's definitely slower than a “human wave.”

Activity 3-1: Picture a Nerve Cell

Introduction

What do you think a nerve cell looks like? Do all nerve cells look and function the same way? In this activity you will explore the structure of a neuron and how it functions. You will make a model of a neuron to help you picture in your mind just how a nerve cell looks and works.

Materials

Markers, crayons; colored pencils

Construction paper

Activity Report

Procedure

Step 1 Draw a picture of a neuron on a piece of notebook paper or colored paper.

Include the following parts and label them.

axon

dendrites

cell body

nucleus

You may use Figure 3.1 as a guide. Be sure that the neuron covers the entire width of the paper with the ends touching the edges of the paper. Be creative in your drawing. For example, you can include as many dendrites as you want. You may want to use yarn or other materials to represent parts of the neuron.

Step 2 Picture the direction a nerve impulse travels as you draw your neuron. Which path do you think a nerve impulse would take from your neuron to another neuron next to it? Draw arrows on your drawing to indicate the direction of the nerve impulse.

Step 3 Place your paper next to your neighbor's and be sure the edges do not touch each other. This space or gap between neurons is called the synapse (SIHN-apps). A nerve impulse must travel from one neuron to another across a synapse. At the synapse, chemicals called neurotransmitters (NUR-oh-trans-MIT-urs) are released from the axon of one neuron. The neurotransmitters travel across the gap to the dendrites of the next neuron to canyon the nerve impulse. Just as there are many types of neurons, there are many kinds of synapses. Different synapses can use different molecules as neurotransmitters.

Step 4 Look at Figure 3.3. You can see two neurons with a small space between them, which are similar to your paper models. Read Steps (a) through (d) for a closer look at how nerve cells work.

a. Dendrites are extensions of the nerve cell membrane. They are the nerve cell's receivers or antennae. They receive messages from receptors and other nerve cells and send them toward the cell body.

b. Information received by the cell body is then passed to the axon.

c. The axon sends the impulse away from the cell body to the other end of the neuron. So if dendrites are like receivers, then axons are like transmitters that send out information to other neurons.

d. The space between two neurons is called the synapse. Chemicals called neurotransmitters pass the information from the axon end of one neuron to the dendrite end of the next neuron.

Some poisonous snakes have venom that can paralyze a person. How do you think the venom affects the nervous system?

What kinds of symptoms do you think a person would have if the axons of his or her nerve cells were damaged?

Activity 3-2: Drug Effects on Neurons

Introduction

The nervous system is built of neurons. They carry electrical messages throughout the body. Neurons pass messages to other neurons and to other cells by releasing chemicals called neurotransmitters. Drugs can disturb the functions of neurotransmitter's. Neurotransmitters are released from the end of one neuron and travel across the gap (synapse) between neurons. Drugs can have many different effects on these events at synapses. You explore some of those effects in this activity.

Materials

Signs that read

Message Sender

Message Receiver

Dopamine Pump

Cocaine

Tennis balls (about 20 to 30)

2 chairs

Scissors

Yarn or string

Resources 1, 2, and 3

Activity Report

Procedure

Step 1 Read Resource 1.

Step 2 Look at Resource 2, Normal Transmission of Dopamine, as your teacher explains the process. Make sure you understand how dopamine is transferred from one neuron to another neuron across the synapse.

Step 4 Your group will be assigned a specific classification of drugs (stimulant or depressant). Discuss the following questions in your group.

Is the drug a stimulant or a depressant?

Use your text to determine what effect this drug type has on a neuron.

Does the drug speed up or slow down the transmission of dopamine? Explain.

Step 5 Your group will teach the rest of the class what you have learned. Create a demonstration that illustrates the effect a specific drug has on the neurons. Decide what roles are necessary to successfully demonstrate the idea. Make your performance interesting for the class. Be creative. For example, your presentation can include music and the script can be a song, poem, or role-play.

Review Questions

Sample answers to these questions will be provided upon request. Please send an email to teachers-requests@ck12.org to request sample answers.